Keels of early sailing ships were strong structural beams, usually made of wood, which extended in a straight line along at the bottom of their hulls. An example of this type of keel is the long keel incorporated in the yacht, Cygnet, shown in FIG. 1. Ballast, often rocks, was located inside the bottom of the hull, distributing its load along the keel's length. A rudder was mounted near the vertical rear end of the hull. Upwind performance was poor, reaching performance was useful. Structures were simple, loads were low.
Higher performance sailing yachts were developed in the nineteenth and twentieth centuries capable of improved upwind performance. This was possible with long keels of more advanced shapes capable of providing efficient hydrodynamic side loads and more effective righting moments. These new features were needed to oppose the large aerodynamic side force and heeling moments developed by the sails when sailing upwind. Some examples of the designs developed by naval designers over the years will be reviewed.
Long keels of curved planform and increased depth were used by Herreshoff's Gloriana in 1891, FIG. 2, and in the more recent 12-meter Columbia in 1958, FIG. 3. These boats retained a keel structurally integral with the hull, but placed the ballast concentrated near the bottom of the deeper keel, increasing its righting moments. The structural simplicity to support ballast is evident from FIG. 2. Control was achieved with an inclined rudder attached at the rear end of the keel. The rudder, having to support only its own loads on a long hinge, was a simple mechanism. Mast and sail position was not too sensitive, because of the long keel and large lateral area. Nevertheless, a reduction of wetted area was achieved with the curved planform, relative to Cygnet of FIG. 1.
Yachts with a shallower displacement hull, shaped independently of the keel, more recently called canoe, use fin keels. Examples are Herreshoff's Wenonah of 1892, FIG. 4, and the more recent Olympic class Soling, FIG. 5. The fin keels are usually made of lead, located near the center of the boat to provide two basic functions: hydrodynamic side force below the sail, and ballast to prevent excessive heel when sailing upwind. This type of yacht uses a separate conventional rear rudder located at the rear of the canoe for steering. These rudders turn on a cantilevered post. Mast and sail position are more critical than in boats with long keels. The fin keel and rudder are also called appendages to the canoe.
A modified approach of fin keel design has been tried in recent decades with large ballast concentration in torpedo-like bulbs or bullets located at the bottom of, and external to, the fin keel which can be made of cast iron. These ballast bodies are longer than the width of the fin keel itself, as, for example, in the Tempest shown in FIG. 6. This boat also uses a separate rear rudder. One disadvantage shown in FIG. 6 is that for a given overall draft 1, the span or vertical dimension 2 of the fin keel is obviously reduced by distance 3 due to the presence of the large bulb 4. Also, the structural thickness and shape of the fin keel has to be large enough to support the side loads due to the weight of the bulb when the yacht heels.
The development of aeronautical technology has made available new design approaches for improving fin keels, improving the two basic functions of the fin keel, which are retained. Some examples are listed below.
A trailing edge flap, first developed for aircraft wings by de Havilland in the 1920's, was added to the trailing edge of the fin keel of the 12-meter boats, for example, Intrepid and Australia II. For example, flap 5 in FIG. 7 enhances side force. Steering of these boats was attained with a separate rudder 6 also shown in FIG. 7. When sailing upwind, the pressure differences between the leeward side and the windward side of the fin keel also causes a hydrodynamic side force which also helps to oppose the sail's side force. These boats won the America's Cup in 1972 and 1983, respectively. All subsequent successful 12-meter boats have used fin keels with flaps.
More recently, winglets, first developed by NASA's Dr. Whitcomb as a device conceptually different from endplates, was successfully incorporated by Sloof and others at the bottom of a fin keel of a 12-meter yacht Australia II, shown as device 7 in FIG. 7. A lead winglet is especially effective, since it obviously lowers the center of mass of the fin keel, but unlike the bulb of the Tempest, it increases the effective span of the fin keel when sailing heeled upwind. This type of keel, which was used by the winning boats of the 1983 and 1987 America's Cup, permits effective maneuvers with a conventional rear rudder.
Another interesting example of aeronautical influence on keels is the Collins' fin keel, conceptually related, according to Collins, to the aeronautical "joined wing." The Collins' keel is sketched in FIG. 8 from data described in the October 1986 SEA HORSE magazine. A lead bulb is attached at the bottom of the fin keel to increase ballast's righting moments, decreasing, for a given draft, the span of the fin keel, as was the case for the Tempest. Collins' fin keel itself, however, is slotted at the middle. The slot induces, according to Collins, a re-distribution of vortex flow which is beneficial for the hydrodynamics side force of the device. Yaw is obtained with a rear rudder 9.
Additional information on the Collins keel is available in U.S. Pat. No. 4,920,906, in which Collins teaches and claims a close coupling between the front and rear members of his keel, between which the slot is formed. Collins teaches that there should be a crossflow from the high pressure side of the forward member across the slot and the low pressure side of the rear member, that the front member should be smaller than the fixed portions of the rear member, and that the distance between his forward and rearward keel members is less than the chord of the rear member.
Other interesting examples of different designs related to keels and rudders are now reviewed.
FIG. 9 shows the 1974 12-meter Mariner using a fin keel for ballast and side force, and a deep bustle for expected hydrodynamic benefits using a blunt end not unlike those used in the aerodynamics of bullets and cars. Control for the Mariner was provided by a narrow rear rudder of small area and very high aspect ratio which protrudes below, instead of to the rear of the bustle. It therefore differs from Australia II's rudder, shown in FIG. 7. Mariner was reported to be difficult to maneuver.
FIG. 10 shows a 12-meter designed by I. Howlett, sketched from a 1977 issue of SEA HORSE magazine. It uses a fin keel to provide side force and ballast. Steering is obtained with an under-slung rear rudder of high aspect ratio very similar to the Mariner. The design also shows a front foil of high aspect ratio and considerable depth. While Howlett's fin keel design of FIG. 12 apparently has been tank tested, it was not used in his 12-meter designs which have been challengers to the America's Cup before or after the publication of that article.
The above designs appear to cover extreme breadth of configuration, but upon analysis, respond to and share fundamental design features which may be summarized as follows:
(a) Keels provide hydrodynamic side force to oppose the sail's aerodynamic side force, and righting moments through ballast to oppose, when heeled, the sail's heeling moments, to permit upwind sailing; and,
(b) Steering is provided by a separate rudder.
In consequence:
(c) The fin keel is of relatively large dimensions and is an important structural component, usually made of lead or cast iron, which supports large hydrodynamic and gravitational loads when heeled.
(d) The rudder is of relatively small dimensions and supports only its own loads.
(e) Mast and sail positions have been evolved over 100 years to define a well-proven criteria in which there is close proximity between fin keel's forward edge and the mast, and a large distance between the fin keel, mast, and rudder. The latter usually at the extreme rear end of the hull.
A completely different kind of yacht has been considered in the past, reproduced in FIG. 11 from a 1903 publication on a model yacht Gossoon. The hull of FIG. 11 comprises a canoe 10 with front foil 11 and rear foil 12 supporting a ballast body 13. According to the original Gossoon drawings, the vertical distance between the belly of the Gossoon's canoe and the top of the ballast is approximately the same as the average horizontal chord distance between the leading and trailing edges of either the front or rear foil of Gossoon and the vertical depth of his ballast body is almost the same as the draft of his canoe. In the horizontal direction, the spacing between the front and rear foils is about 4 times the horizontal chord of the foils. According to Marchaj's "Aerohydrodynamics of Sailing", Appendix 2, this type of design has been tried in an experimental yacht in 1968 and in models without noticeable success. Marchaj's picture of a wood model shows a vertical distance between belly of the canoe and top of hull approximately equal to twice the horizontal chord of either of his foils, and a horizontal distance between the foils of approximately three times the chord of the rear foil, which is the larger one. Marchaj's picture of a quarter-ton experimental yacht, apparently unsuccessful, shows that the vertical distance between the ballast body and the canoe body is also approximately twice the horizontal chord of the rear foil, and the horizontal space between the foils is also approximately three times, or three and one half times, the chord of the rear foil. This lack of success can be understood upon examination of FIG. 11, inasmuch as the previously reviewed design criteria for conventional yachts (paragraphs (a) through (e) above) do not apply to FIG. 11. Indeed, the word keel, denoting a single appendage that provides two functions (hydrodynamic side force and ballast) is not properly applicable to FIG. 11, because the foils of FIG. 11 are not ballast, since body 13 is the ballast. Also, if foil 11 were a "rudder", it would exhibit structural problems never experienced by earlier rudders, namely, the need to support about half of the weight of ballast 13. And, if it is a rudder, it is not seen how the small front foil can provide adequate forces against the sail's side force.
It is then clear that new design problems appear for FIG. 11, which conventional design criteria does not address: How can the configuration of FIG. 11 sail upwind? Where should its mast be positioned? How can a hydrodynamic side force on rudder-like foils oppose sail's side force and provide simultaneous steering? How should the structure be constructed if large ballast weights are to be supported by rudder-like foils? The absence of these type of boats in general use indicates they appear to have no purpose, or no solution has been found to these problems.